Stability - basic information, links

[dougfrolich]

Stability is so fundamental to boat design, a dedicated stability thread makes sense to me--here is some info, not new but usefull, to mabey kick things off.

http://www.radford-yacht.com/stablty1.html

[Vega]

Stability is so fundamental to boat design, a dedicated stability thread makes sense to me--here is some info, not new but usefull, to mabey kick things off.

www.radford-yacht.com/stability1.html

Yes I agree, but I can not open your link:(

[dougfrolich]

Sorry Vega try it now.

[Guillermo]

An interesting information on fishing vessels capsizes, just in a small part of the world:
http://www2.worksafebc.com/i/posters/2003/capsize_vessel.htm

[Vega]

Looks like this is the stability thread so, take the time to download this:

http://web.usna.navy.mil/~phmiller/offshore.ppt#1

Nothing new,and quite simple but a very well made document (and a very complete and nice one) about stability and general hull form (sailboats).

[dougfrolich]

Nice presentation--but I have to take issue with what is depicted in slide 14--there is no reason why a "beamy" must have a higher CG ( represented by W in the slide )
Slide 5 shows an open 60 inverted--too much faith was put in its high initial stability at the expense of its total range of possitive stability, and its high degree of inverted stability was ignored. We all know that bulb weight in that example was reduced way too much to reduce overall weight--That is what happens when you go to extreams
Going back to slide 14; if you build the wide boat light, yet strong, and add the saved weight in the from of a bulb, enough so that the VCG is in the same possition as on the narrow boat, then your overall stability behavior will be better on the beamy boat--especially if you camber the deck, and can count on the intact volume of the coach roof, also intellegent use of water ballast can increase initial stability, and roll inertia. 2cents

[Vega]

Nice presentation--but I have to take issue with what is depicted in slide 14--there is no reason why a "beamy" must have a higher CG ( represented by W in the slide )


I think that you misunderstood what he is saying. He says that to have equal righting arms in those two boats (the narrow one and the beamier) the narrow one will have to have a lower CG. And that is so, because the beamier boat can add a lot more form stability while the narrow one will have to rely a lot more on the ballast, though the lower CG (see slide 15).


Going back to slide 14; if you build the wide boat light, yet strong, and add the saved weight in the from of a bulb, enough so that the VCG is in the same possition as on the narrow boat, then your overall stability behavior will be better on the beamy boat--…

I agree in what concerns initial stability (the beamier boat will be faster and can carry more sail) but in what concerns negative stability, the beamier boat will also have a lot more inverted stability ( its form stability is even better with the boat inverted). For that reason the narrow boat will recover a lot faster from a capsizing. Roll and motion comfort would also be a lot worst on the beamier boat, we can see why on the slide 18.

To decide who has the best overall stability, will depend on what you value most: speed or safety and comfort.;)

[Guillermo]

A very interesting presentation on fishing vessels stability.
http://www.shipmotion.se/Download/Guide%20to%20Stability.pdf

[mflapan]

Some very interesting links. Thank you.

Guillermo, the last link did not seem to work. Any ideas?

Regards
Mori

[Guillermo]

Mori,
Here you have the document in PDF
Cheers.

[Guillermo]

Another link:
http://hawaii-marine.com/templates/stability_article.htm
Cheers.

[mflapan]

Guillermo

Thank you for the file regarding fishing vessel stability. I will read it with interest. As part of the work that I am doing on the section for stability tests and stability information, I am considering the function of stability documentation. The following is proposed wording from the standard.

"The stability documentation shall incorporate the following functional documents:
a) A stability compliance report prepared for the purpose of confirming the vessel’s compliance with the relevant stability criteria and to allow independent verification if required.
b) An operator’s stability manual containing the information needed to safely operate the vessel in respect of its stability."

I am not sure that the two functions are necessarily always well served by being incorporated in the same document, particularly with smaller vessels where the competencies of the operator might not cover the skills needed to investigate and interpret stability calculations. I notice that the USCG require stability letters for certain vessels that are posted in the wheelhouse. Also that the Marine and Coastguard Agency in the UK have published generic stability manuals for certain types of vessels less than 24 metres in length. Then there is the possibility of adopting electronic formats for use by the operator which allow complex calculations to be undertaken without the operator fully understanding them.

I would be interested to hear of your views and those of other participants to this forum as to the most effective formats for stability documentation.

Best regards
Mori

[Guillermo]

Mori,
I think the idea of providing two separate documents, one for the technicians and other, more simplified and easy to understand, for the captains and crew, is the way to go.
I'm collaborating with SMC systems (http://www.shipmotion.se/) on the developing of a computerized system able to provide the fishing captains with real time information on their vessel's stability. You may be interested in contacting Helge Vestin, their chairman, for more information.
I attach an interesting document from the Wolfson Unit on the matter (regarding fishing vessels).
Cheers.

[mflapan]

Dear Guillermo

Thank you for the report and the contacts for computerised stability analysis. The latter is certainly a good way to go.

One problem that fishing vessels face is that they load at sea in conditions that can be difficult to read drafts for the purposes of stability analysis. Have you any thoughts on this?

Regards
Mori

[Guillermo]

Mori,
SMC are working on devices and programs able to permanently and accurately measure draft(s) when at sea, whatever the load condition. I think the system will be a great tool for fishermen (and not only).
Cheers.

[Guillermo]

Mori,
Something from the Wolfson Unit:

"Development of Safety Guidance for Fishermen

Over the past 3 years the Wolfson Unit has been working on a number of contracts for the Maritime and Coastguard Agency to develop a simple method of safety guidance. The fishing industry has a relatively poor safety record, but proposals for increased regulation consistently meet with resistance from the industry. An alternative approach has now been proposed by the Wolfson Unit, to provide each vessel with a single page Stability Notice which will help fishermen to judge the level of safety of their vessel in relation to the sea conditions that they are operating in. Rather than penalise some vessels, it will advise fishermen of the limitations of their craft in terms of stability.

The guidance will include information on loading of the vessel, and lifting of heavy loads over the side. These notices are not intended to give precise predictions of when a capsize might occur, but to encourage the fishermen to become more aware of the hazards and the degree to which they affect safety. For the large vessels with stability booklets the guidance is based on an assessment of the residual stability when loaded or when lifting. For the thousands of small vessels with no stability information the guidance is based on the residual freeboard, a parameter which the fisherman should be able to estimate. For these small vessels the freeboard guidance is based on the length and beam of the vessel, with no expensive surveys or calculations required. It is hoped that these Stability Notices will become commonplace in the near future, and will help to improve the safety culture.

Whilst it has been developed for the UK fleet, the method is considered relevant to the worldwide fishing industry, in particular to address the very poor safety record within fleets of small vessels in developing countries. The concept was introduced to delegates at the IMO SLF meetings in July and aroused considerable interest. A paper describing the method will be presented by Barry Deakin at the STAB 2006 conference in Rio de Janeiro in September."

Cheers

[CDBarry]

Note that there is a Coast Guard NVIC and an ABS guidance document on the preparation of stability data. NVIC 5-86 also contains guidance on stability data presentation.

[kach22i]

Some great links and downloads in this thread, thanks.:)